Selectively operable multiple loads



s. A. CIOTTI ET AL 3,436,528

'SELECTIVELY OPERABLE MULTIPLE LOADS A ril 1, 1969 Filed Oct. 22, 1965 DC. POWER INVENTORS GEGRGF A. C/o'rr/ Kara 14 flora: s

United States Patent U.S. Cl. 235-92 Claims ABSTRACT OF THE DISCLOSURE This disclosure relates to an electrical interlocking circuit for interconnection of a plurality of key lock units to a gasoline dispenser and a plurality of counters. Each switch circuit includes a silicon controlled rectifier having a self-triggering circuit and a separate holding circuit. The several counters have one side connected directly to the related individual control circuit such that each counter is operated through the corresponding silicon controlled rectifier and only one is operated at any given time. A disconnect circuit is also connected between the holding circuit and each of the trigger circuits to disable all other trigger circuits in response to establishment of one holding circuit. The holding circuit is controlled by a time delay circuit actuated by the output of the flow responsive means and by a common input to the self-triggering circuit.

This invention relates to selectively operable multiple loads and particularly to apparatus for selectively actuating a plurality of counters and the like in response to a coded input and including means to restrict actuation to a single counter at any given time.

In the bulk dispensing of gasoline and the like, a coded control system may be provided to provide unattended stations wherein a truck driver enters and loads his truck without requirement of a supervisory attendant. Such systems however require that the control system be constructed to record the withdrawal and to prevent fraudulent or erroneous recording as a result of malfunctioning. Highly developed key control systems have been provided; for example, as disclosed in the copending application of T. H. Houle et al., entitled, Dispensing and Metering Control, filed on July 6, 1964 with Ser. No. 380,379, now Patent No. 3,252,620, and assigned to the same assignee as the present application. The latter application discloses a key control system whereby each authorized driver or customer is provided with his own key operated lock in the control unit. When the truck driver enters the station, he controls the system by operating of the proper key lock. The turning of the key not only conditions the system for dispensing of the product to the tank truck but simultaneously provides for insertion of the proper customer related counter into the circuit of a transmitter coupled to the flow line such that the withdrawal is recorded in that counter. An interlock is provided to restrict operation of the system to a single key at any given time and therefore recording on a single counter. This prevents fraudulent or erroneous counting on more than one counter and each customer will only be charged for the gasoline actually Withdrawn by an authorized truck driver or the like.

The present invention is particularly directed to an electrical interlocking circuit for interconnecting of key lock units or other coded control means such that only a single counter can be connected into the circuit at any given time and the counter is related to the actuated lock unit. The circuit of the present invention provides a rela tively inexpensive solid state circuitry having a high degree of reliability. The circuit can be operated without a regulated power supply and may include a novel failsafe feature such that the whole system looks out and requires resetting by the superintendent or other personnel of the bulk plant in the event of malfunction of the operating components.

Generally, in accordance with the present invention, a control circle is provided for each customer having a suitable on-ofi" device such as a key lock-operated switch. The switches selectively connect each of the individual customer control circuits to a common output control circuit and an individual recording circuit. The conditioning is effected by interconnecting each of the counters for operation through a separate part of the control circuit such that all of the counters are conditioned for operation, but only the circuit or the counter connected with the actuated control circuit can be operated. When a switch is actuated to complete the circuit connections, the output control circuit permits dispensing of a product and the counter circuit conditions the appropriate counter to record the flow.

Thereafter, the flow generates a signal which is simultaneously applied to all of the counters. However, only the counter operably connected in the circuit will be actuated by the signal.

Further, in accordance With another aspect of the present invention, all of the control circuits are so actuated after the initial establishment of a single control circuit that closing of the related key for any other control circuit will be ineffective to actuate that control circuit. Consequently, only one key is effective to operate the discharge control. Additionally, a safety interlock is provided such that in the event of malfunctioning with two or more of the control and therefore recording circuits turned on, a disconnect device or the like is energized.

In a practical and novel construction of the present invention, each of the individual control circuits includes a controlled or triggered rectifier such as a silicon controlled rectifier having a self-triggering circuit connected in parallel with the main circuit of the controlled rectifier. When the corresponding switch is closed, the control circuit is completed to first establish a trigger pulse in the trigger circuit which fires the rectifier which thereafter maintains a conduction condition as long as the related switch is controlled or a separate holding circuit is maintained. A transistor or the like has its input circuit connected across a portion of the control circuit to the common side of the switches such that when any of the switches are closed to fire the related silicon controlled rectifier the transistor is biased to conduct. The main discharge control and the counter circuits are connected in parallel to the output circuit of the transistor. The several counters are connected in parallel with each other having an individual side connected directly to the related individual control circuits such that each counter is operated through the corresponding silicon controlled rectifiers. Consequently, when a silicon controlled rectifier is fired to bias the transistor on, only the related counter is operably connected in a completed circuit through the transistor and the silicon controlled rectifier.

A current circuit is interconnected between the power supply and each of the silicon controlled rectifiers such that once a silicon controlled rectifier is fired, the holding current circuit provides sufficient current to the silicon controlled rectifier to maintain conduction thereof after the switch is opened. A disconnect circuit is also connected between the holding circuit and each of the trigger circuits to disable all trigger circuits in response to establishment of the holding circuit. The holding circuit is controlled by a time delay circuit actuated by the output of the flow responsive means. The related silicon controlled rectifier is held in the on condition after the related switch has been opened for a short period to record the discharge of liquid in the line between the pumping means and the metering means which is transferred to the tank truck or the like.

A fail-safe control unit is connected in series with a portion of the holding circuit. If more than one controlled rectifier is turned on, the total current flow through the holding circuit is increased to a level to trigger the fail-safe control unit which is actuated to lock out the flow system and require resetting by supervisory personnel.

The drawing furnished herewith illustrates a preferred embodiment of the present invention in which the above advantages and features are clearly disclosed as well as others which will be clear from the following description of the drawing.

The drawing is a schematic circuit diagram of a control circuit interconnected with a gasoline or other fluid dispensing unit which is diagrammatically shown.

In the drawing, a bulk storage tank 1 is provided hav ing a discharge conduit or line 2. A pump 3 is connected in line 2 and connected to a suitable motor 4 or the like for operation. A control unit 5 for the motor 4 is provided at a central supervisory location or the like. A main control relay 6 includes a set of normally open contacts connected as a part of the control unit 5 as diagrammatically illustrated. The control relay 6 is connected in a key actuated selection circuit 7 of the present invention to permit operation of the motor 4 whenever any one of similar switches 8 and 9 is closed by actuation of a corresponding key operated lock 10 and 11. The locks 10 and 11 are diagrammatically shown as they may be formed of any suitable construction including the well known tumbler type locks adapted to receive a unique key 12 which when turned to an unlocked position closes the corresponding switch 8 or 9, generally as shown in U.S. Patent 2,777,555. Although only a pair of key actuated switches and locks has been illustrated for purposes of describing the present invention, in actual practice a great number of key locks can be selected and connected into the circuit in the manner of the two assemblies illustrated and hereinafter described.

Generally, the selector circuit of the present invention includes similar individual control branches 13 and 14 interconnected to a set of power lines 15 through a series connected key lock switch 8 or 9, respectively. Each of the branches 13 and 14 is similarly constructed and branch 13 is hereinafter described with corresponding elements in the second branch 14 identified by corresponding primed numbers.

When the switch 8 is closed, the control branch 13 is triggered to complete a circuit between the power lines 15 and provide a turn-on bias to a control transistor 16. The relay 6 is connected in the output circuit of the transistor 16 and is energized to close the control contacts 61 and allow the operation of the motor 4 and pump 3 whenever transistor 16 conducts.

When the relay contacts 6-1 are closed, the motor 4 operates the pump 3 and permits discharge through the line 2. A metering means 17 has a volumetric flow driven meter 18 connected in the discharge side of the pump 3 to record the liquid pumped from the storage tank 1 through the discharge line 2 to a suitable receptacle such as a tank truck or the like, not shown. A transmitting unit 19 is formed as a part of the metering means 17 and is suitably connected to meter 18 to establish a series or train of signal pulses. The transmitting unit 19 is shown as a meter driven switch 20 connected by leads to a pulse forming circuit 21. The output of circuit 21 is a train of pulses at a repetition rate corresponding to the flow through the meter 18. The output of circuit 21 is connected to a time delay means 22 and to the one side of each of a pair of counters 23 and 23'. The energization circuits of the counters 23 and 23' are separately completed through the control branches 13 and 14, respectively, and consequently only the counter associated with the actuated switch is connected to record the flow.

The time delay means 22 is connected to transistor 16 and to the pulse forming circuit 21 for conjoint control. The means 22 may be of any suitable construction. A unique timing circuit suitable for such use is shown in the copending application of T. H. Houle entitled Data Acquisition Monitoring System which was filed on July 6, 1964 with Ser. No. 380,400 and is assigned to the same assignee as this application.

The time delay circuit 22 includes an output relay 24 having a set of normally open contacts 24-1 connected in an interlock branch line 25 which connects each of the control branches 13 and 14 to the power lines 15 such that triggering of one branch disables the triggering circuits of all branches and also provides a timed holding circuit for the fired branch 13 or 14. The disabling of the triggering circuits prevents the erroneous or fraudulent firing of the other branch and connection of the second counter 23' in an operative circuit when counter 23 has been previously connected. The holding circuit maintains the triggered branch circuit operative for a selected period after opening of the key lock switch 8 to record the final fluid flow through the meter 18 of the fluid between the meter 18 and the pump 3.

A fail-safe relay 26 is connected to the power lines 15 through a transistor 27 and the time delay contacts 24-1. Relay 26 controls a set of normally closed contacts 26-1 connected in circuit with the main relay 6. The transistor 27 has its input circuit connected across a portion of the holding circuit. If both of the branches are conducting for any reason, the current through the holding circuit increases to a level to bias the transistor 27 on and thereby actuate the fail-safe relay 26.

The fail-safe relay 26 can be of any suitable construc tion and preferably includes means which requires manual resetting of the relay such that supervisory personnel are required to reset the system for subsequent delivery. This permits maintaining of a record of any erroneous or fraudulent malfunctioning of the system.

More particularly, in the illustrated embodiment of the invention, each of the key lock switches 8 and 9 has one side connected in common by a jumper lead 28 and to the positive power lead 15 in series with a pair of resistors 29 and 30. The opposite contact of the switches 8 and 9 is connected in series to the corresponding branches 13 and 14 of which branch 13 is specifically described.

Branch 13 includes a silicon controlled rectifier 31 having a cathode 32 connected to the negative power line 15 and its anode 33 connected in series with a blocking diode 34 to the one contact of the key lock switch 8.

The silicon controlled rectifier 31 further includes a trigger gate 35 connected to a trigger line 36 for firing of the silicon controlled rectifier when the related switch 8 is closed. The trigger line 36 includes a resistor 37 connected in series with a capacitor 38 and a blocking diode 39 in parallel with the main circuit of the silicon controlled rectifier 31 and the diode 34. A resistor 40 is connected in parallel with the capacitor 38.

In operation, when the related switch 8 is closed, power is applied across the anode to cathode circuit of silicon controlled rectifier 31 which is selected to remain nonconducting in the absence of a trigger signal at gate 35. Current simultaneously flows through the diode 39, capacitor 38 and resistor 37 to the negative power line and rapidly generates a firing signal at the gate 35 which causes the silicon controlled rectifier 31 to turn on and provide a path directly from the switch 8 to the negative line 15 which is maintained until the anode to cathode circuit is broken.

The current flowing through the bias resistors 29 and 30 provides a voltage signal for turning on the control transistor 16.

The illustrated control transistor 16 is shown as a PNP type connected in a common emitter configuration with a base 41 connected to the junction of the bias resistors 29 and 30, an emitter 42 connected directly to the positive power line and collector 43 connected to one side of the control relay 6. The opposite side of the relay 6 is connected to the negative power line 15 in series with the relay contacts 26-1 of the tail-safe relay 26. A suitable protective diode 44 is connected in parallel with the relay 6 in accordance with known circuit connections. When the switch 8 is closed and the silicon controlled rectifier 31 fired, the current through the bias resistor 30 generates a bias across the base to emitter junction of the transistor 16 to cause it to conduct. This of course provides current through the emitter to collector circuit of transistor 16 to energize the relay 6 which closes its contacts 6-1 in the control unit 5 to energize the motor 4.

The collector 43 of the transistor 16 also is connected to the time delay circuit 22 via a line 45. The time delay circuit 22 energizes the relay 24 and relay contacts 24-1 close to complete the holding circuit to rectifier 31 and a disconnect circuit to branch 14, and condition relay 26, as follows.

The holding circuit for the rectifier 31 includes the time delay contacts 24-1 connected in series with a common resistor 46 and an individual diode 47 in series with an individual resistor 48 to the related anode 33 of the silicon controlled rectifier 31. The diode 47 is polarized to conduct in the same direction as the silicon controlled rectifier 31 and consequently provides a holding circuit generally in parallel with switch 8. Diode 34 prevents current flow from the holding circuit through the trigger circuit in the standby position. A corresponding holding circuit for rectifier 31' of branch 14 includes a diode 49 and resistor 50 connected in series between common resistor 46 and the anode 33 of rectifier 31.

The disconnect circuit to branch 14 is as follows:

The normally open relay contacts 24-1 have one side connected directly to the positive power line 15 and the opposite side connected by individual resistor 51 to the junction of the diode 39' and the capacitors 38' of branch 14. Consequently, when the time delay contacts 24-1 close, a circuit is completed to the capacitor 38' of the branch 14. This charges the capacitor 38' to its full charge but resistor 51 prevents firing of the silicon controlled rectifier 31'. Consequently, actuation of the related switch 9 at this condition is ineffective to cause triggering of silicon controlled rectifier 31' and completion of the circuit for counter 23.

A similar resistor 52 is connected between the line 25 and the junction of diode 39 and capicator 38 to disable branch '13 if switch 9 has been previously closed.

The several counters 23 and 23' have their one side connected in common via line 53 to the output of the pulse forming circuit 21 which in turn is connected to be energized in accordance with the operation of the transmitter 19 which includes the switch which is periodically opened and closed. This opens and closes the input circuit of a suitable pulsing circuit; for example, as shown in the previously referred to application, which generates a train of pulses in accordance with the switch operation. The train of pulses is therefore directly related to the volumetric flow through the meter 18 and line 2. The pulse train is simultaneously applied to both counters 23 and 23'. Counters 23 and 23' may be of any suitable variety responsive to a pulse input and are shown in block diagram with similar visual indicators. However, the circuit of each of the counters 23 and 23 is connected in series with individual control diodes 54 and 55 to the corresponding branches 13 and 14. In particular, the diode '54 is connected to the junction of the anode 33 of silicon controlled rectifier 31 and the diode 34. Diode 55 is similarly connected to the junction of anode 33 and diode 34'. A silicon controlled rectifier 31 or 31' must be conducting before the related counter 23 or 23' can be actuated by the incoming pulse train at line 53 from the pulse forming circuit 21.

Referring again to the triggered branch 13, the rectifier 31 is of course maintained energized as long as the related switch 8 is closed or the time delay contacts 24-1 are closed. Thus, at the end of a delivery or whenever switch -8 is opened, the rectifier 31 is controlled by contacts 24-1 or relay 24. The pulse train is also applied to circuit 22, as shown, to actuate the circuit 22 and hold relay 24 energized. The circuit deenergizes relay 24 if a pulse is not received within a selected period. When the flow is terminated, pulses are no longer fed to the counter 23 or time delay circuit 22. After the timing period, the time delay circuit 22 deenergizes the relay 24. Contacts 24-1 therefore open and break the holding circuit for the related rectifier 31 as Well as to reset the triggering circuits by allowing capacitors 38 and 38' to discharge through resistors 40 and 40'.

The fail-safe relay 26 is connected in circuit with the transistor 27 between the negative power line 15 and the time delay contacts 24-1. The fail-safe transistor 27 shown as a PNP type is connected in a common emitter configuration with a base 56 connected to the junction of the resistor 46 to the diodes 47 and 49, with an emitter 57 connected to line 25 and particularly the time delay contacts 24-1 and a collector 58 connected to relay 26. When a single rectifier 31 is conducting, the holding current through resistor 46 creates an insufficient voltage drop across the resistor 46 to turn on the trasistor 27. As a result the relay 26 is de-energized. However, if both the rectifiers 31 and 31 conduct, the sum of the holding currents creates an increase voltage drop across resistor 46 to a level sufiicient to bias transistor 27 to conduct and energize relay 26. If relay 26 is energized, the contacts 26-1 open and break the circuit to the main relay '6. Consequently, the main control contacts 6-1 open and disable motor 4. As previously noted, relay 26 can be made with a mechanical latch such that it must be reset by supervisory personnel or the like.

The operation of the illustrated embodiment of the invention may be briefly summarized as follows.

When a customer desires to withdraw gasoline, the proper key operated switch 8 or 9 is actuated to close the circuit to the related control circuit. If switch 8 is closed, current through capacitor 38 and resistor 37 generate a gate pulse for firing the related silicon controlled rectifier 31. The current through the resistors 29 and 30 creates a bias on the control transistor 16 and turns it on, thereby simultaneously supplying power to the main motor relay 6 for starting the motor-pump unit and to the time delay circuit 22 to operate relay 24. The time delay contacts 24-1 close and create a current through the resistor 51 and 52 to charge the capacitors 38 and 38 to full voltage such that the triggering circuits are inoperative until such time as the circuits are reset.

The time delay contacts 24-1 also establish the holding circuit through the resistor 46 and related diode 47 and resistor 48 to the silicon controlled rectifier 31.

Consequently, the driver can then withdraw gasoline or the like from the storage tank. All of the flow from the storage tank 1 is through the meter 17, and consequently, the transmitter 19 is driven in synchronism and in accordance with the total flow. The transmitter 19 actuates the pulse forming circuit 21 to generate a pair of pulse trains directly related to the total volume of flow through the meter. The synchronous pulse trains are simultaneously applied to the counter 23 and 23' and to the time delay circuit 22. The application to the time delay circuit 22 is of no particular significance so long as the related switch 8 is closed to maintain control transistor 16 conducting.

The application of the pulse train to the counters 23 and 23' however is effective to record the quantity of fluid flow on the related customer counter 23 with the circuit being completed through the associated diode 54 and silicon controlled rectifier 31. The flow is not recorded on counter 23' because rectifier 31 is cut off or nonconducting.

When the delivery is completed the motor-pump unit 7 is turned off by opening of the switch 8 which removes the bias from the control transistor 16 and thereby removes power from the control relay 6 and also from the time delay circuit 22.

After the motor 4 has been turned off, the liquid in the line 2 between the pump 3 and the meter 17 is under sufficient pressure to cause it to drain through the meter 17 and the discharge end of line 2. This quality of liquid should be recorded. The transmitter 17 continues to apply the signals to the pulse forming circuit 21 and maintains the two synchronous pulse trains. The train applied to the time delay circuit 22 maintains the relay 24 energized independently of power from line 45 thereby maintaining the time delay contacts 24-1 closed as long as pulses of a minimum repetition rate are applied to circuit 22. Consequently, the holding circuit for the related silicon controlled rectifier 31 is maintained during the final flow of the liquid from pump 3 through meter 18. Consequently, the counter 23 is maintained active and records the final liquid flow. When pulses cease, the time delay circuit 22 times out and opens the time delay contacts 24-1. This opens the holding circiut to the silicon controlled rectifier 31 and resets the rectifier to the standby position. The capacitors 38 and 38' discharge through the paralleled resistors 40 and 40' and the circuit is reset for a subsequent operation.

If for any reason more than one of the silicon controlled rectifiers 31 or 31' is simultaneously operated as a result of malfunctioning or the like, the total current flow through the resistor 46 turns on transistor 26 and energizes the relay 26 and actuates the lock out contact 26-1.

The present invention thus provides a relatively inexpensive and reliable means for discriminating between the several customers and their keys and provides a means to insure proper recording of the dispensing of gasoline products and the like.

We claim:

1. An operating control circuit for selectively actuating a plurality of devices from a common drive, comprisindividual control branches, one for each of said devices, each of said branches including a switch means and means for operating the switch means in response to power across said branch,

a plurality of coded switches, one for each device and connected between the corresponding branch and a common circuit terminal for selective application of power,

control means connected in circuit with the common circuit terminal and responsive to conduction through a branch circuit for actuating the common drive, and

an interlock means connected to the control branches and having an electrically actuated input means responsive to energization of one branch to disable all other branches.

2. The operating control circuit of claim 1 having a time delay means responsive to opening of the coded switches for actuating the interlock means to reset all branches to standby.

3. An operating control circuit for selectively actuating a plurality of devices from a common drive, comprising individual control branches, one for each of said devices, each of said branches including a switch means and means for operating the switch means in response to application of power to the control branch,

a plurality of coded switches, one for each device and connected between said branches .and a common circuit termnal for selective power application to the branches,

control means connected in a circuit with the common circuit terminal and responsive to conduction through a branch circuit for actuating the common drive,

means for separately connecting each of said devices for operation through the related branch circuit 8 whereby only one device is actuated at any given time, and

an interlock means connected to the control branches and having an electrically actuated input means responsive to energization of one branch to disable all other branches.

4. The operating control circuit of claim 3 having a holding circuit means connected to the interlock means and said control branches to maintain a triggered branch operable independently of the related coded switch.

5. The operating control circuit of claim 4 having a fail-safe means responsive to the current in the holding circuit means to disable the system if more than one branch is maintained operable.

6. A operating control circuit for selectively actuating a plurality of devices, comprising a common drive for said devices,

a plurality of paralleled individual control branches, one for each of said devices, each of said branches including a coded switch means in series with a triggered switch means and trigger means for triggering the triggered switch means in response to a selected input power across the paralleled branches,

input power means,

a bias means connected in series with the paralleled branches to the input power means,

an electronic amplifying means having an output circuit and having an input circuit connected to the bias means and biased to conduct in response to current through the bias means,

a common drive control means connected in the output circuit for actuating the common drive, and

means for connecting each of the devices in a separate operating circuit through the related control branch whereby only one device is responsive to operation of the common drive.

7. The control circuit of claim 6 having gate controlled rectifiers as having power eletrodes connected in series with the coded switch means and a gate electrode,

a capacitive firing circuit connected across the power electrodes, a holding circuit connected to said power electrodes and to the input power means,

a switch serially connected in said holding circuit, and means connected to operate the switch and responsive to conjoint operation of the common drive and the common drive control means.

8. A recording system for individually recording the sequential delivery of petroleum products through a common line to different customers, comprising a plurality of counters, one for each customer,

a single flow responsive device having means to generate a pair of pulse trains in accordance with flow through said common line, said means being connected to said counters to simultaneously apply the one pulse train to all counters,

a control means for controlling flow through said common line,

a plurality of coded switches, one for each counter,

a control circuit including a plurality of branch circuits, one for each of said counters, said branch circuits being parallel connected and each branch including a corresponding coded switch connected in series with a controlled rectifier, each branch including a capacitive firing circuit connected to fire the associated controlled rectifier in response to a selected current therethrough, said control circuit having .a common impedance means connected in common between the paralleled branches and a power input means,

a transistor connected with the control means in a power circuit and having an input circuit connected to the impedance means and biased to conduct in response to current flow through the common impedance means and any one of the branch circuits,

a time delay means having timing contacts and connected in a parallel circuit with the control means for simultaneous energization thereof, said timing contacts being held in the actuated position for a selected period after deenergization of the time delay means,

means connecting the second pulse train of the flow responsive device to said time delay means and operable to maintain the delay means energized independently of the control means, and

an interlock circuit including the timing contacts and individual paralleled 'branch circuits to each of the firing circuits and individual paralleled holding branch circuits to each of the controlled rectifiers.

9. The recording system of claim 8 having a fail-safe means connected to disable the control means, and

a transistor connected in series with the fail-safe control means and the delay contacts, the transistor having an input circuit connected to respond to the current flow through the holding branch circuits and biased to conduct in response to conduction through more than one of said holding branch circuits. 10. A recording system for individually recording the sequential delivery of petroleum products through a common line to different customers, comprising a plurality of counters, one for each customer,

a single flo-w responsive means having means to generate a pair of similar pulse trains in accordance with fiow through said common line, said means being connected to said counters to simultaneously apply the one pulse train to all counters,

a control means for controlling flow through said common line,

a plurality of key actuated switches, one for each counter,

a control circuit including a plurality of branch circuits, one for each of said counters, said branch circuits being parallel connected and each branch including a corresponding key actuated switch connected in series with a controlled rectifier having main electrodes and a gate electrode,

each branch including a capacitive firing circuit connected to fire the associated controlled rectifier in response to a selected current therethrough, said control circuit having voltage dividing resistors connected in common between the paralleled branches and a power connection means,

diode means connecting the counters to the corresponding branch circuits for energization through the corresponding controlled rectifier whereby a counter is actuated by the pulse train from the flow responsive means only when the associated controlled rectifier is conducting,

a transistor having output elements connected in a power circuit with the control means and having input elements connected to the common voltage dividing resistors and biased to conduct in response to current flow through the resistors and any one of the branch circuits,

a time delay circuit having a first and a second input and an output relay having a set of contacts, the first input being connected to the output of the transistor power circuit for energization and the second input being connected to the flow responsive means and actuated by the second pulse train, said delay circuit energizing the relay to immediately actuate its contacts and holding the contacts actuated a selected period after de-energization at both said inputs,

an interlock circuit including said contacts and individial resistors connected at least one each between the contacts and each firing circuit for charging said firing circuit and thereby disabling said firing circuits,

diode means in the firing circuit to prevent bypass of power from the interlock circuit through the main electrodes,

a holding circuit including said contacts and a bias resistor in series with individual holding branches, each branch including a diode in series with a resistor connected between said bias resistor and the controlled rectifier of the related branch to provide a holding current independently of the corresponding key actuated switch,

a disconnect transistor having an input circuit connected across the bias resistor and an output circuit connected in series with the relay contacts, said transistor being biased to conduct in reponse to current through the bias resistor greater than a single branch holding current circuit, and

a disconnect relay connected in the output circuit of the disconnect transistor and having contacts connected in the power circuit for the control means to disable said power circuit.

References Cited UNITED STATES PATENTS 3,252,620 5/1966 Houle 2222 MAYNARD R. WILBUR, Primary Examiner. GREGORY J. MAIER, Assistant Examiner.

U.S. Cl. X.R. 

